I’m fascinated by these sorts of mass delusions. They seem almost laughably strange – hundreds of people convince themselves they are sick – until you realize that collective hysteria is only the flip-side of the placebo effect. We are all capable of talking ourselves into feeling better so it only makes sense that we are also capable of talking ourselves into feeling worse. (In this sense, most alternative medicine is just another form of mass delusion. We confuse our expectations with reality.) Needless to say, this is yet more evidence of Descartes’ Error: the mind and body are inseparable.

The teenage girls hobbled into a prayer meeting at their Roman Catholic boarding school, their knees buckling with every step. For months, a mysterious illness had swept through their school, afflicting hundreds of girls, and they were there to ask for recovery.

The first isolated cases of the illness, which affected the girls’ walking and made them feverish and nauseated, appeared in November and December. After the girls came back from Christmas break, the illness spread. By February, the school’s director, Sister Margie Cheong, had become alarmed and alerted the authorities.

At the Wednesday prayer meeting, Sister Michaela Shim handed out cookies and began to tell a story in her Korean-accented Spanish. The girls laughed and shouted as her improvised parable unfolded. It was the story of a boy who lies to get attention.

What may be happening here is far more complex. After batteries of tests, doctors now believe that the illness that has struck 600 of the 3,600 girls at this charity-run school is psychological.

In medical terms, Mexico’s public health authorities have concluded that the girls at the Children’s Village School are suffering from a mass psychogenic disorder. In layman’s language, they have a collective hysteria.